Metallising pastes

ABSTRACT

A metallising paste, suitable for forming an adherent electrically-and thermally-conductive metal-containing deposit on a ceramic surface and having one application in providing screenprinted microcircuit conductors and a further application in providing heat abstracting pads, or &#39;&#39;&#39;&#39;heat sinks,&#39;&#39;&#39;&#39; applied to the underside of microcircuit substrates, comprises a heatvaporizable liquid medium containing as powder a glaze material and a component selected from aluminum or an alloy thereof, wherein the glaze dissolves the oxide of aluminum or aluminum alloy and wets ceramic surfaces at the fusion temperature of the glaze. Preferably the glaze fuses at a temperature not greater than the melting temperature of the component.

United States Patent [191 Davey METALLISING PASTES [75] Inventor: NormanDavey, Newbury, England [73] Assignee: United Kingdom Atomic EnergyAuthority, London, England 22 Filed: Apr. 28, 1972 21 Appl. No.: 248,670

[30] Foreign Application Priority Data May 10, 1971 Great Britain14091/71 [52] US. Cl. 117/227, 117/160 R, 252/512, 252/514 [51] Int. ClB44d l/l8 [58] Field of Search 117/227, 201, 160 R;

[4 1 Apr. 30, 1974 Primary Examiner-Cameron K. Weiffenbach Attorney,Agent, or Fi,rmLarson, Taylor and Hinds 57] ABSTRACT A metallisingpaste, suitable for forming an adherent electrically-andthermally-conductive metalcontaining deposit on a ceramic surface andhaving one application in providing screen-printed microcircuitconductors and a further application in providing heat abstracting pads,or heat sinks," applied to the underside of microcircuit substrates,comprises a heatvaporizable liquid medium containing as powder a glazematerial and a component selected from aluminum or an alloy thereof,wherein the glaze dissolves the oxide of aluminum or aluminum alloy andwets ceramic surfaces at the fusion temperature of the glaze. Preferablythe glaze fuses at a temperature not greater than the meltingtemperature of the component.

25 Claims, No Drawings METALLISING PASTES BACKGROUND OF THE INVENTIONDESCRIPTION OF THE PREFERRED EMBODIMENTS The nature of the presentinvention is further ex- This invention relates to metallising pasteswhich 5 plained, by way of example, by reference to the followformadherent electrically-and thermally-conductive metal-containing depositson ceramic surfaces when fired in contact therewith and to methods forforming conductive elements on ceramic surfaces with said pastes: theinvention has one application in forming microcircuit conductors onceramic substrates and a further microcircuit application in providingpads adherent to the underside of ceramic substrates for abstractingheat from the substrates by thermal conduction.

SUMMARY OF THE INVENTION According to the present invention ametallising paste suitable for forming an adherent electrically-andthermally-conductive metal-containing deposit on a ceramic surfacecomprises a heat-vaporizable liquid medium containing as powder a glazematerial and a component selected from aluminum or an alloy thereof,wherein the glaze dissolves the oxide of aluminum or aluminum alloy andwets ceramic surfaces at the fusion temperature of the glaze.

By dissolving the oxide of aluminum or an alloy thereof the fused glazerenders soluble at least part of the insulating oxide film known to coatthe metal particles; both the electrical and thermal conductivity of thedeposit are thereby substantially increased. Preferably the glaze fusesat a temperature not greater than the melting temperature of thecomponent: it has been found that if the glaze fuses at a temperaturegreater than the melting temperature of the component i.e. the glazefuses after the component is molten, the conductivity of the depositformed is lowered.

The preferred glaze for use with aluminium or an aluminium alloycomprises an oxide of boron: the glaze may additionally comprise anoxide of lead. A suitable liquid medium for use in a metallising pastecomprising aluminium or an aluminium alloy and the preferred glaze is apolymerized cyclic ketone dissolved in a solvent such as terpineol. A-suitable ceramic surface for use with such an aluminium-containingmetallising paste is alumina. Surfaces other than alumina which may beused include silica or alumino-silicate surfaces. Aluminium alloys whichmay be used in pastes according to the present invention include Al/Cuand Al/Ag alloys.

The invention also provides a method for forming an electrically-andthermally-conducting element on and adherent to a ceramic substratecomprising forming a replica of said element on said substrate using ametallising paste as afore-defined, drying the formed replica, firingthe dried replica at a temperature not less than that at which the wholeof the replica fuses, whereby the replica wets the substrate and isrendered conducting, and solidifying the fired replica to form asubstrateadhering electrically-and thermally-conducting element. Thereplica may be formed on the substrate by screen-printing. I

In addition the invention provides a ceramic substrate having anelectricallyand thermally-conducting element formed thereon by themethod of the present invention.

ing embodiments thereof.

A heat-vaporizable liquid medium was mixed with aluminium powder, boricoxide and lead oxide to form a paste having a consistency suitable forscreenprinting microcircuitry. The liquid medium was a polymerizedcyclic ketone dissolved in terpineol, a suitable medium being Engelhard4/575 (supplied by Engelhard Industries). A suitable paste contained 35ccs of liquid medium for each gms of solids consisting of 92 gms Al, 6gms PhD and 2 gms B 0 The preferred particle size of the Al powder was1-2 p. and of the lead and boric oxides was 400 mesh. The paste wasscreenprinted onto an alumina substrate to produce a replica of amicrocircuit comprising a plurality of conducting elements. Thesubstrate used was a 96% A1 0 material supplied by Worcester PorcelainCo. The substrate, with replica printed thereon, was dried at betweenlO0-200C (to remove the vaporizable material) and was then fired in airat 830C at which temperature both the aluminium powder and the mixedboric oxide/lead oxide are molten. The substrate was fired for 50 mins.The molten aluminium powder formed a conductive microcircuit replica andthe molten boric oxide/lead oxide wetted the substrate by interfacialreaction therewith. On subsequent cooling to ambient temperature asolidified and electrically-conductive micro circuit was formed whichwas bonded to the substrate by the solidified mixed oxide glaze. Theelectrical conductivity of microcircuitry formed from air-fired replicasas aforesaid is typically 20-30 mQ/square (a square being 1 mm across)compared with, for example, values of 4 mQ/square for Au, 10-15mQ/square for Pt/Au and 60 mQ/square for Pd/Ag. Although the metalpowder in the above embodiment is Al, powders of Al alloys may be used,in particular powders of Al/Cu and Al/Ag alloys.

Apart from providing microcircuit electrical conductors the inventionprovides heat-abstracting elements bonded to the underside ofmicrocircuit substrates for the removal of heat therefrom. Suchelements, usually in the form of pads and termed heat sinks, can also bedeposited as paste replicas by screen-printing and converted tothermally-conducting pads by firing and subsequent solidification. It isknown in the art that the dissipation of the heat generated by highdensity microcircuitry presents a problem: one method of substantiallyreducing substrate heat is to form thermally conducting elements (heatsinks) according to the present invention.

1 claim:

1. A metallising paste suitable for forming an adherent electricalconductor on a ceramic surface consisting essentially of aheat-vaporizable liquid medium, a glaze powder and a component in powderform selected from Al or an alloy thereof, wherein the glaze dissolvesthe oxide of said component and wets ceramic surfaces at the fusiontemperature of the glaze.

2. A metallising paste as claimed in claim I wherein the glaze fuses ata temperature not greater than the melting temperature of the component.

3. A metallising paste as claimed in claim 1 wherein the component isselected from the group consisting of an Al/Cu and Al/Ag alloy.

4. A metallising paste as claimed in claim 1 wherein the liquid mediumis a polymerized cyclic ketone dissolved in a solvent.

5. A metallising paste as claimed in claim 1 wherein the glaze iscapable of wetting a ceramic surface selected from the group consistingof alumina, silica and aluminosilicate.

6. A metallising paste as claimed in claim 1 wherein the glaze comprisesan oxide of boron.

7. A metallising paste as claimed in claim 6 wherein the glazeadditionally comprises an oxide of lead.

8. A metallising paste as claimed in claim 1 wherein the component is Almetal.

9. A metallising paste as claimed in claim 8 wherein the glaze comprisesan oxide of boron.

10. A metallising paste as claimed in claim 9 wherein the glazeadditionally comprises an oxide of lead.

11. A metallising paste as claimed in claim 9 wherein the liquid mediumis a polymerized cyclic ketone dissolved in a solvent.

12. A metallising paste as claimed in claim 9 wherein the glaze iscapable of wetting a ceramic surface selected from the group consistingof alumina, silica and aluminosilicate.

13. A method for forming an electricallyand thermallyconducting elementon and adherent to a ceramic substrate comprising forming a replica ofsaid element on said substrate with a metallising paste as claimed inclaim 1, drying the formed replica, firing the dried replica at atemperature not less than that at which the whole of the replica fuses,whereby the replica wets the substrate and is rendered conducting, andsolidifying the tired replica to form a substrateadheringelectrically-and thermally-conducting element.

14. A method as claimed in any of claim 13 whereby the replica is formedon the substrate by screenprinting.

15. A method as claimed in claim 13 wherein the glaze in the paste fusesat a temperature not greater than the melting temperature of thecomponent.

16. A method as claimed in claim 15 wherein the component in the pasteis selected from the group consisting of an Al/Cu and Al/Ag alloy.

17. A method as claimed in claim 15 wherein the glaze in the pastecomprises an oxide of boron.

18. A method as claimed in claim 17 wherein the glaze additionallycomprises an oxide of lead.

19. A method as claimed in claim 17 wherein the liquid medium in thepaste is a polymerized cyclic ketone dissolved in a solvent.

20. A method as claimed in claim 17 wherein the glaze in the paste iscapable of wetting a ceramic surface selected from the group consistingof alumina, silica and aluminosilicate.

21. A method as claimed in claim 15 wherein the component in the pasteis Al metal.

22. A method as claimed in claim 21 wherein the glaze in the pastecomprises an oxide of boron.

23. A method as claimed in claim 22 wherein the glaze additionallycomprises an oxide of lead.

24. A method as claimed in claim 22 wherein the liquid medium in thepaste is a polymerized cyclic ketone dissolved in a solvent.

25. A method as claimed in claim 22 wherein the glaze in the paste iscapable of wetting a ceramic surface selected from the group consistingof alumina, silica and aluminosilicate.

2. A metallising paste as Claimed in claim 1 wherein the glaze fuses ata temperature not greater than the melting temperature of the component.3. A metallising paste as claimed in claim 1 wherein the component isselected from the group consisting of an Al/Cu and Al/Ag alloy.
 4. Ametallising paste as claimed in claim 1 wherein the liquid medium is apolymerized cyclic ketone dissolved in a solvent.
 5. A metallising pasteas claimed in claim 1 wherein the glaze is capable of wetting a ceramicsurface selected from the group consisting of alumina, silica andaluminosilicate.
 6. A metallising paste as claimed in claim 1 whereinthe glaze comprises an oxide of boron.
 7. A metallising paste as claimedin claim 6 wherein the glaze additionally comprises an oxide of lead. 8.A metallising paste as claimed in claim 1 wherein the component is Almetal.
 9. A metallising paste as claimed in claim 8 wherein the glazecomprises an oxide of boron.
 10. A metallising paste as claimed in claim9 wherein the glaze additionally comprises an oxide of lead.
 11. Ametallising paste as claimed in claim 9 wherein the liquid medium is apolymerized cyclic ketone dissolved in a solvent.
 12. A metallisingpaste as claimed in claim 9 wherein the glaze is capable of wetting aceramic surface selected from the group consisting of alumina, silicaand aluminosilicate.
 13. A method for forming an electrically- andthermally-conducting element on and adherent to a ceramic substratecomprising forming a replica of said element on said substrate with ametallising paste as claimed in claim 1, drying the formed replica,firing the dried replica at a temperature not less than that at whichthe whole of the replica fuses, whereby the replica wets the substrateand is rendered conducting, and solidifying the fired replica to form asubstrate-adhering electrically-and thermally-conducting element.
 14. Amethod as claimed in any of claim 13 whereby the replica is formed onthe substrate by screen-printing.
 15. A method as claimed in claim 13wherein the glaze in the paste fuses at a temperature not greater thanthe melting temperature of the component.
 16. A method as claimed inclaim 15 wherein the component in the paste is selected from the groupconsisting of an Al/Cu and Al/Ag alloy.
 17. A method as claimed in claim15 wherein the glaze in the paste comprises an oxide of boron.
 18. Amethod as claimed in claim 17 wherein the glaze additionally comprisesan oxide of lead.
 19. A method as claimed in claim 17 wherein the liquidmedium in the paste is a polymerized cyclic ketone dissolved in asolvent.
 20. A method as claimed in claim 17 wherein the glaze in thepaste is capable of wetting a ceramic surface selected from the groupconsisting of alumina, silica and aluminosilicate.
 21. A method asclaimed in claim 15 wherein the component in the paste is Al metal. 22.A method as claimed in claim 21 wherein the glaze in the paste comprisesan oxide of boron.
 23. A method as claimed in claim 22 wherein the glazeadditionally comprises an oxide of lead.
 24. A method as claimed inclaim 22 wherein the liquid medium in the paste is a polymerized cyclicketone dissolved in a solvent.
 25. A method as claimed in claim 22wherein the glaze in the paste is capable of wetting a ceramic surfaceselected from the group consisting of alumina, silica andaluminosilicate.